Yes! Most lenders will gladly talk to you to help you start planning a purchase. The best thing to do is have a conversation with a broker or lender (or a few brokers or lenders), see who you like working with and get a rough idea of how much you would qualify for based on verbal information. This is easier for someone who earns a fixed salary, no overtime, no complicated pay structures, not a business owner, etc.
You can also fill out an application, submit your documents (income, assets, etc), and get pre-approved. This will allow you to shop for properties more easily, and will also make it easier to run different scenarios for different properties that you view.
The best way to prepare is to have your income for the last two years and YTD available, know how much you have available to spend, and write down any questions you want to ask.
They should be able to give you a rough idea of an interest rate (of course interest rates change daily) based on estimated credit score, purchase price, down payment amount, location, loan type, property type, etc.
All of this is necessary before looking at properties because you need to know how much you can borrow, how much you need to put down, and approximately how much you will make from each potential rental.